Best RPM for most efficient fuel consumption.

Plus the A/C take about 5HP... The alternator takes some more, esp if you have 350W worth of stuff plugged in as one guy does (refer to the alternator post going on right now too). Add some more stuff for power steering and smog devices and then you've got more loss.

Those numbers come close to agreeing with a test Serge Scherbatskoy (a consultant where i used to work) did in his old pickup truck. He built an accelerometer and went 2 directions on the highway at 55mph. He took his foot off the accelerator and his device captured the instantaneuos deceleration. he got 12hp for his number.(I asked the same question you did, the other hp is used fighting inertia) It was some old full size truck, no cap, I guess tailgate up, etc. I think edmund scientific may have a gizmo that does this so , if you're better at math than me, you could figure it out for yourself. Since Serge worked on the Manhattan Project and even shared a room with Enrico Fermi - I guess it was easy for him!

Carl

thank you - this is a fun exercise!

Carl

Thank you for holding up my end of the argument while I was away for a weekend ski-trip :-)

Those figurs are clearly much better than we get from our actual cars, so if there is a constant "load" or loss somewhere to add to the aeerodynamic drag then it must be higher than this. How much higher?

I don't know figures for fuel consumption per hp, but by reverse-engineering your results I get MPG = 16.35 * MPH/HP.

I think it gets very interesting if you work out the numbers for HP+20 (i.e. asssuming 20 hp losses in engine, drivetrain etc):

MPH HP MPG MPG(HP+5) MPG(HP+20)

10 0.6 273 29 7.9 20 1.6 204 49 15.1 40 5.9 111 60 25.3 60 15.8 62 47 27.4 70 23.7 48 40 26.2 80 33.9 38 33 24.3

Empirically, this looks pretty darn close to the sort of mileage (US mpg) a lot of us get out of our Legacy's at constant speeeds on the open road. (for non-US people, those numbers are 9.5, 18.2, 30.3, 32.9,

31.4, 29.1 mpg)

Note that the most economical speed is 60 mph, not 40 mph. And in fact

70 mph is 4% more economical than 40 mph! (and 80 mph is only 4% worse than 40 mph!)

Here is another cross-check. Earlier this year I spent several weeks driving a Ford Territory around suburbs at as close to a constant 40 km/h as wee could manage (measuring 3G mobile phone data rates for acceptance testing by a telco). Over that time, according to the trip computer, we averaged 21 l/100km (13.5 mpgUK, 11.2 mpgUS). At one point I was able to drive at a very steady 110 km/h (70 mph) for 60 or 70 km on a straight & flat road and it acheived 12 l/100 km (23.6 mpgUK, 19.7 mpgUS). That's a ratio of 1.75 between the two speeds. Looking at the numbers above, we see a ratio between 70 mph and 20 mph of 26.2/15.1 =

1.735. That's very much in line with each other.

My math isn't up to par here, but the data point that would fall at 50 MPH would seem to be important on this curve. (big gaps in the data, and the speed falls directly into the range in question) And, if you do HP+30, I'm sure your number might show the best mpg is at 75...IOW you could factor this into anything you want. ;-) There's no way the car chews up 20 hp internally anyway...it moves when I let go of the brake at idle, when I'm generating what...a couple hp?

-John O

I agree. I think there is much less than a 5 hp loss from piston to rubber.

20 at a minimum, and then it goes up from there. Look at a typical motorcycle magazine -- they will often compare RWHP (rear-wheel hp) to crank HP and the difference is much larger than 5. Hell, even on a motorcycle it can be as much as 20 hp, and those things are a lot less complex (well, they used to be anyway!) and have much less accessories like A/C and stereos systems to drive (minus the gold wing, but hell, that bike even has reverse!).

mike

I meant much *more* not less. But I see where JonO is going. To do this correct you'd need real numbers, both RWHP and rank HP from a car, with known consumption rates to back up the math. But I seems like we are for sure headed in the right direction.

Well, yes and no. it's true that if you try HP+32 (30 isn't quite enough) then 70 mph takes the lead as the most efficient speed. The problem with that is that it's only 20.5 mpgUS (24.7 mpgUK) which is well under the numbers we actually see from our cars at those speeds.

You only have a few surplus hp at idle -- that is, power available to move the car -- but how much is being chewed up in addition in internal friction and also in driving the alternator and aircon compressor? Aircon on makes a very noticable difference to the acceleration or hill climbing ability of cars with engines under about 1500cc so that must take quite a bit of power. And yet the engine can power it at idle.

If the car *doesn't* chew up 20 hp internally then why don't we get 50 -

60 mpg? Or at least 40 - 45?

It's a percentage loss, not a specific quantity, at least to a good approximation. An adequate reference is at:

It states that a typical loss through the drive train is around 12%, which seems to match fairly well with other references I've seen. So, if the required horsepower for the speed is 20, the power output required from the motor is about 23, rounded off.

You've not indicate how you arrived at those figures but it appears you are assuming 100% energy content of the fuel is used to move you down the road. Thermal efficiency of a gas internal combustion engine is on the order of 25% with the remaining energy being lost in the form of heat, cooling system, exhaust, radiation etc.

Mickey

Yep, all of that makes good sense. Then, when combined with the gasoline engine pumping losses (smaller throttle openings yield less efficiency), it is usually true that the best fuel mileage for a vehicle occurs at the lowest speed at which top gear can be used. (lowest engine rpm/friction loss, and greatest throttle opening for the required output)

Of course, that's usually too low a speed to be practical for most highways...

personally I travel at ~80km/h when no traffic is around... if I notice that somethings coming from behind, I speed up to 100km/h, at 80km/h I get almost 7.5l/100km, at 100 it is closer to 9l/100km.

"" wrote: > In article , > "JohnO" wrote: > > > > I think it gets very interesting if you work out the > numbers for HP+20 > > > (i.e. asssuming 20 hp losses in engine, drivetrain etc): > > >

this doesnt take any real math to figure out. my escort gets its best milage at 50mph my concorde gets its best milage at 65mph. and yet still a pruis gets its best milage between 10-15mph with multi stops. there is no golden speed that if we all travel at we get the best milage. its as simple as: the best mpg is gotton when your car travels the most miles on the least gas. all the math in the world wont really help you because there is to mch to take into account, air pressure, incline, road surface, weight, physical location on the globe, temp, elevation, etc, etc, etc. and just forget about the car itself, current oil displacement, tire pressure, tire tread, gearing, etc, etc, etc, etc, etc, etc. I could fill a dictionary sized book with just the lists of things you would have to take into account, plus you would have to take it all into account atleast every tire rotation. to have any real arguement you have to use the KISS rule (keep it simple stupid)

Wind resistance? Maybe it's that air scoop on the hood, of the fog lamp sockets. Ahhh, whatever. On my way in to work today, I was driving 47 mph at about 1600-1700 rpm. The engine is barely working, and I'd love to be able to drive at this speed for a couple hundred miles to see what happens.

-John O

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I have to get up to about 53 mph first, and then back into ~45. That's about as slow as I can go without the trans shifting to a higher gear.

-John O

Thx for the topic and a lot of interesting knowledge. I was wondering what is the function MPG(MPH) too for long time. I will add 2c too, something was surprised me about year ago. My wife took het Imreze 2.5TS and went alone around the half USA about year ago. Chicago-New Orleans-Mexico Gulf-Texas-New Mexico-Arizona-Nevada-Arizona-Utah-Colorado-Nebraska-Chicago) trip yields 10k miles. I flew in and we met in Las Vegas and drive back to home together.

I was observing the MPG for long time and at speeds 80-90 MPH we were getting about 25MPG. Once we went into the Colorado mountains the MPG raised about 2MPG yet having similar speeds. I was shocked ? I thought the mountains should affect MPG negatively. I do not have an explanation, but I kind of think it is due to more efficient engine works, at full throttle and full torque, what is often a case in mountains.

A.